On Tuesday, an offshoot of Anonymous, called AntiSec, claimed to have purloined a list of 12 million iOS unique device identifiers, or UDIDs, along with various bits of user data connected to those UDIDs. While the revelation raised the specter of privacy violations, it became clear that users have plenty of questions about what kind of information can and cannot be gleaned from one’s iPhone or iPad UDID. Here, we explain what exactly a UDID is, how it is used, why Apple deprecated its use by developers, and why the privacy fears aren’t entirely unfounded.